1. Field of the Invention
The present invention relates generally to an apparatus and method for controlling a traffic rate in a mobile communication system, and in particular, to an apparatus and method for controlling a reverse traffic rate in a mobile communication system.
2. Description of the Related Art
Commonly, a mobile communication system utilizing Code Division Multiple Access (CDMA) supports a multimedia service using the same frequency band. In the CDMA mobile communication system, a plurality of users can simultaneously transmit data, and the users are identified using unique codes assigned thereto.
In the system, reverse data is transmitted over a packet data channel on a physical layer packet (PLP)-by-PLP basis. In this case, a packet length is fixed, but a data rate is variable for each packet. Therefore, information on a mobile station, such as power and the amount of transmission data of the mobile station, is fed back to a base station. Based on the feedback information, the base station controls a rate of transmission packet data by scheduling.
A process of determining a data rate of a mobile station, which varies for each packet, in the manner described above is called “scheduling,” and the scheduling is performed by a scheduler of a base station. The scheduler of a base station performs scheduling based on a Rise of Thermal (RoT) indicating ‘thermal noise-to-total reception power’ or a load obtained from RoT of a mobile station belonging to a base transceiver station (BTS). A scheme in which a base station controls a reverse rate of a mobile station includes a Limited Rate Transition scheme. In the Limited Rate Transition scheme, a base station limits a transition of a data rate to one step.
A description of a general Rate Transition scheme will be made under the assumption that a possible set or table of data rates includes 9.6 Kbps, 19.2 Kbps, 38.4 Kbps, 76.8 Kbps, 153.6 Kbps, and 307.2 Kbps in order. In addition, it is assumed that a rate of packet data that is transmitted in a reverse direction by a mobile station at a particular time is 38.4 Kbps. The data rate set or table is subject to change in number and values of data rates included therein according to different systems. The Rate Transition scheme can be classified into a Full Rate Transition scheme and a Limited Rate Transition scheme.
In the Full Rate Transition scheme, a base station can set all the data rates for a next packet of a mobile station. That is, in a system using the Full Rate Transition scheme, a mobile station can change its current data rate of 9.6 Kbps to 307.2 Kbps at once. Therefore, a reverse rate of a mobile station, which can be allowed by a base station, is not limited from a previous rate of the mobile station. However, in the Limited Rate Transition scheme, a base station limits a transition of a data rate to one step in determining a data rate of a next packet of a mobile station. For example, the base station restrictively sets a rate of the next packet to one of 38.4 Kbps, 76.8 Kbps, and 153.6 Kbps for a mobile station that currently transmits data at 76.8 Kbps. In other words, the base station enables the mobile station to increase or decrease its current data rate of 76.8 Kbps by one step, or hold the current data rate. Accordingly, the transition of a data rate of a mobile station is limited.
The Limited Rate Transition scheme is disadvantageous in that a base station restrictively changes a data rate of a mobile station by one step. However, the Limited Rate Transition scheme is advantageous in that the scheduling result can be transmitted with a single bit, thereby contributing to minimization of overhead. In addition, the Limited Rate Transition scheme shows a relatively small variation in interference to other cells by limiting the transition of a data rate of a mobile station to one step.
A description will now be made of operations of a base station and a mobile station, information transmission, and channels for the information transmission in the current Limited Rate Transition scheme.
When RoT is available, a scheduler in a base station performs scheduling such that the RoT should be set to a reference RoT level. However, when RoT is not available, the scheduler performs scheduling such that the RoT should be set to a reference load level. Based on the scheduling result, the base station transmits a rate control bit (RCB) to a mobile station. The RCB is transmitted to a particular mobile station over a forward rate control channel (F-RCCH). The names of a specific control bit and a specific channel used herein are given for the convenience of explanation, by way of example.
A mobile station increase a data rate in a next transmission period by one step if an RCB value received from a base station is (+1) (Up), and decreases the data rate in the next transmission period by one step if the RCB value is (−) (Down). In addition, if the received RCB value is ‘0’ (Maintain, or Hold), the mobile station maintains a data rate of a previous transmission period.
FIG. 1 is a timing diagram illustrating a procedure for controlling a data rate of a mobile station using a Limited Rate Transition scheme according to the prior art. In FIG. 1, an RCB is transmitted from a base station to a mobile station over F-RCCH 101 for each transmission period. The RCB, as described above, is used by a base station to control reverse rates of mobile stations. A reverse link in FIG. 1 includes a reverse packet data control channel (R-PDCCH) 104, a reverse packet data channel (R-PDCH) 105, and a reverse pilot channel (R-PICH) 106. The R-PDCCH 104 is a control channel, which is transmitted together with the R-PDCH 105, and transmits a rate indication sequence (RIS) for relating a rate of data transmitted over the R-PDCH and a mobile status sequence (MSS) for relating power and buffer status of a mobile station. It should be noted herein that the types of information sequences and the number of the information bits, which are transmitted over the R-PDCCH 104, are changeable according to systems. Table 1 below shows an RIS field of R-PDCCH and R-PDCH data rates assigned thereto.
TABLE 1RIS in R-PDCCHR-PDCCH Data Rate00000Kbps00019.6Kbps001019.2Kbps001138.4Kbps010076.8Kbps0101153.6Kbps0110230.4Kbps0111307.2AKbps1000307.2BKbps1001460.8Kbps1010614.4Kbps1011768.2Kbps1100921.6Kbps11011.024Mbps
As shown in Table 1, if a value of the RIS field is ‘0001’ (RIS=0001), the R-PDCH is transmitted at 9.6 Kbps. Other sequences are also interpreted in the same manner. In addition, it should be noted that value of the rates shown in Table 1 are subject to change according to different systems.
The MSS contains status information of a mobile station, and the status information is transmitted from the mobile station to a base station. More specifically, the mobile station generates an MSS relating whether the mobile station desires to increase, maintain, or decrease a data rate in the next transmission period, taking the amount of data stored in its buffer and current transmission power thereof into consideration. Thereafter, the mobile station reports the generated MSS to the base station. Here, it should be noted that the mobile station cannot immediately determine its data rate based on the MSS report, but the data rate should be allowed by a scheduler in the base station. A detailed description thereof will be given below. Table 2 below shows an example of the MSS.
TABLE 2MSSMeaning00Rate Up requested by MS01Rate Down Notified by MS10Rate Maintain Requested by MS11Unused
As shown in Table 2, if a value of MSS is ‘00’ (MSS=00), a mobile station (MS) must increase a data rate in a next transmission period from a data rate in a current transmission period by one step. If a value of MSS is ‘01’ (MSS=01), the mobile station intends to decrease the data rate in the next transmission period from the data rate in the current transmission period by one step. Here, it should be noted that the mobile station sends a notification rather than a request when decreasing its data rate. Even though the mobile station decreases its data rate without a base station's approval, the system is not affected. Further, if a value of MSS is ‘10’ (MSS=10), the mobile station maintains the same data rate in the next transmission period. A definition of MSS=11 is not given.
Now, referring to FIG. 1, a detailed description will be made of a scheme for controlling a rate of a mobile station according to the prior art.
In FIG. 1, data to be transmitted to a base station arrives at a buffer of a mobile station at a time 107. The mobile station starts transmitting the data stored in the buffer at an initial data rate of 9.6 Kbps from a time 108. In FIG. 1, for the initial data rate of 9.6 Kbps, the system permits all mobile stations to transmit data without a base station's approval. It is assumed that transmission power of the mobile station at the time 108 is sufficiently lower than a maximum transmission power limit of the mobile station. At the time 108, the mobile station transmits 9.6-Kbps data over R-PDCH and simultaneously transmits RIS and MSS over R-PDCCH. Because a data rate of R-PDCH is 9.6 Kbps, RIS corresponding thereto becomes ‘0001’ as shown in Table 1. In addition, because the mobile station can transmit data at a data rate higher than 9.6 Kbps, the MSS becomes ‘00’.
Upon receiving the R-PDCH and the R-PDCCH transmitted for one frame at the time 108, the base station performs a scheduling process. In the scheduling process, the base station analyzes MSS=00 received from the mobile station, determines that the mobile station requests to increase its data rate as a result of the analysis, and determines whether it can increase the data rate of the mobile station, considering the reverse signals (i.e., total RoT or reverse load) received from the other mobile stations.
The base station allows the mobile station to increase the data rate as a result of the scheduling, and then generates an RCB according thereto. At a time 102, the base station transmits the generated RCB, thereby indicating ‘Rate Up’ to the mobile station over F-RCCH. At a time 109, the mobile station receives the RCB over F-RCCH and increases its data rate to 19.2 Kbps by one step based on the received RCB. As shown in Table 1, the data rate 19.2 Kbps is higher than the data rate 9.6 Kbps by one step. At the time 109, RIS in R-PDCCH transmitted together with R-PDCH becomes ‘0010’. The base station and the mobile station repeat a series of processes until the mobile station completely transmits the data stored in its buffer.
Through the processes described above, the mobile station can increase its data rate an a one-step (or step-by-step) basis, based on the amount of data stored in its buffer, a ratio of the maximum transmission power limit to current transmission power of the mobile station, and distribution of the total reverse resources by the base station. After transmitting all the data, the mobile station stops data transmission. When data transmission by the mobile station is suspended, R-PDCCH transmits RIS of ‘0000’.
The conventional power control method, as described above, has the following problems.
Although the mobile station can increase/decrease its data rate under a control of the base station, a level of the increment/decrement is limited to one step. This means that it takes quite a long time for the mobile station to reach a high data rate. Therefore, even when a transmission environment is excellent and the amount of data stored in the buffer of the mobile station is large enough, it will still take quite a long time for the mobile station to reach an appropriately high data rate. That is, even when the mobile station stores a sufficiently large amount of transmission data in its buffer and can transmit reverse data at a high data rate and a reverse load is not so high, it takes quite a long time to satisfy a desired high data rate, thereby resulting in a reduction in reverse data throughput of the mobile station and reverse throughput of the entire system.
In the process of controlling a reverse data rate of a mobile station, when the mobile station has no more transmission data, while transmitting data at a high rate, the mobile station abruptly suspends data transmission and then transmits information relating that the data rate is ‘0’, over the R-PDCCH. Therefore, the base station expects the mobile station to continuously receive data, until it receives RIS of ‘0000’ over R-PDCCH from the mobile station. As a result, the base station cannot assign reverse resource for the mobile station to other mobile stations. Accordingly, an unnecessary reverse load of the mobile communication system is previously assigned to a specific mobile station, thereby resulting in a waste of reverse resources and a reduction in reverse throughput.